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I The Supergalactic plane and a coordinate system

  1. Jan 3, 2019 #1
    There is remarkably little information on the internet including Wikipedia on this topic. Can someone point me in the right direction as I want to build a visualization software that illustrates the supergalactic plane and the coordinate system with any kind of celestial sphere involved.

    All I know is that the plane intersects 3 super galactic clusters. That's it! That I got from Wikipedia. But I need more specific information about the coordinate system, such as where does the 0,0 ra and dec reference point to? Or if there is a cartesian coordinate system, where is 0,0,0 referencing?

  2. jcsd
  3. Jan 4, 2019 #2


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    I don't get it. The Wikipedia page on supergalactic coordinate system has reference coordinates for galactic and equatorial coordinates. Have you been looking at a different page, or is that not what you want?
  4. Jan 4, 2019 #3
    But galactic plane is not a point, as a plane intersecting another plane it would show up as a line would it not? How would this be a singular reference point on any celestial sphere?

  5. Jan 4, 2019 #4


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    In a celestial coordinate system you need two points (both given by Wiki) - the 0° point of the equator, and one of the poles - to uniquely draw a line across the entire sky that represents e.g. celestial equator, ecliptic plane, galactic equator, or supergalactic plane. The 0° point in this case is defined as one of two points the observer sees as the intersection between the galactic and supergalactic planes. The line you mentioned goes from 0° to the observer and to 180° on the other side. Meaning, you only need one point w/r to the observer (the origin) to represent this line.

    If you've never had any experience with celestial coordinates, try reading about those on the Wiki (the images help a lot). Playing with some free planetarium software, like Celestia, might be of some use too, as these include options to display a few coordinate systems at the same time (I don't remember seeing the supergalactic plane specifically included anywhere, but maybe just I wasn't looking). So you can see how e.g. equatorial and galactic coordinates look like on the sky when viewed from Earth.
  6. Jan 4, 2019 #5
    Is it possible for you to draw a picture with paint brush or something? I'm having trouble visualizing this. When two planes intersect there will not be 2 points but a straight line shouldn't there?

    I've tried YouTube too but there's no information about this topic there either.

    Or are the two planes intersecting with the observer along the line so the observer is between 0, 180 right ascension equivalent?

    Ah, it is isn't it? I just reread your post , so my next question is what is located at 0,0? what does the observer see if anything? Milkyway's core?

    Also along the north south pole axis is it parallel to anything? I suppose that's unlikely. if it were the galactic plane it would be exactly 90 degree's.But does the north and south poles point to anything?

    And what is the observer? Our sun? Or center of our milky way? Which object at the center of our sphere does it (supergalactic plane) cut though? If anything? Or is the observer just empty space between the super clusters? Where exactly is this observer? If it's empty space then I supposed computers would be needed to calculate its location each time this coordinate system is used by observing the reference clusters and our galactic plane since its not a visible marker in space/sky.
    Last edited: Jan 4, 2019
  7. Jan 4, 2019 #6


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    I can't really draw anything at this moment. But take a look at this gif from the Wiki:
    The origin is always the observer - so typically, us, on Earth (or Sun, it doesn't really matter, in the same way as it doesn't matter for equatorial coordinates whether you're observing stars from high or low latitudes).
    The supergalactic plane is just another such plane bisecting the celestial sphere, with its zero point longitude where it intersects the galactic equator plane (there are two such points on the sphere; I don't know why one was chosen over the other). That's the 0,0 coordinate. I don't think it points to anything special.
    The poles are just at 90 degrees to the plane and again don't point to anything special.

    Here's a map of galaxy distribution as seen from Earth, drawn in the supergalactic coordinates:
    The galactic plane is seen in blue. The supergalactic equatorial plane goes left to right. You can see some concentration of galaxies roughly along this line.
  8. Jan 5, 2019 #7
    Are we (Earth & Sun) considered part of the galactic plane? Or are we slightly above/below it? I'm asking because I want to make sense of your picture. Does the picture show the galactic plane being a 1 dimensional line (because it is perpendicular to us)?

    Also is the intersection occurring right on the galactic core?

    Lastly are we in the supergalactic plane ourselves?
    Last edited: Jan 5, 2019
  9. Jan 5, 2019 #8


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    I think there's something that should have been made clear earlier. There's a difference between the supergalactic plane - the structure, and supergalactic plane - the equatorial plane of the coordinate system.
    The former is a roughly (with emphasis on roughly) pancake-shaped arrangement of nearby galaxies (of which Milky Way is a part), with some spatial extent and shape that is not all that precisely known, and not necessarily all that planar. The latter is a plane bisecting the celestial sphere of any observer, that was chosen to coincide roughly with the plane of the structure (as it was thought to be some years back), and which is precisely defined.

    In the discussion so far, I was focusing on the coordinate system, but I'm starting to think you were thinking of the structure instead.

    The relationship between the two is like the relationship between the plane of the Milky Way and the equatorial plane of the galactic coordinate system. Or the plane of Earth's equator and the equatorial plane of the equatorial coordinate system. You can be some place in space w/r to and you can point to some location of the former, but the latter - while parallel to the physical planes - extend to infinity and pass through the observer (bisecting the celestial sphere in two).

    So, you can say you are somewhere w/r to the galactic plane (above, below), but the galactic plane in the coordinate system passes through the observer. You can say you are somewhere w/r to the plane of the Earth's equator, but the equatorial plane of the coordinate system passes through the observer. Same with the physical supergalactic plane and the equatorial plane of the supergalactic coordinate system.

    It doesn't matter where you are w/r to the physical structure as far as using the relevant coordinate system goes. You can always use those coordinates, even if you're so far away from the structure (e.g. the Milky Way), that you can't see it. At which point it would no longer have much physical significance, and be just a largely arbitrary way of dividing the celestial sphere.

    If you want to read about the physical structure, follow the paper linked to at the bottom of the Wiki page on the coordinate system (here it is for convenience). Note that they're using Cartesian supergalactic coordinates, which have the Z axis pointing from the Earth towards the north pole of the spherical SGP coordinates, and X towards the 0,0 point of the same (Y being at 90 degrees to both).
    They have some helpful planar projections of galactic distributions in Fig.1, and it's worth noting the general 'uh, not that much or a plane, really' conclusion.

    Back to that 2MASS picture - treat the supergalactic plane as if we're being inside another galaxy, only larger. We see the Milky Way as a line on the sky, because we're inside of it. Same with the SGP. Just as you can see the band of the MW angled w/r to the ecliptic at 60-odd degrees, because the Solar System has the planets orbit in a plane at that angle to the galactic plane, you can similarly see the band of the SGP (such as it is) angled w/r to the galactic plane because the plane of galactic rotation is at close to 90 degrees to the SGP.

    The intersection is not at the galactic centre, but at the galactic coordinates given by Wikipedia. So over 137 degrees away from the galactic centre.
    One can imagine seeing the same picture drawn in galactic coordinates (whose 0,0 point is towards the centre).
    It'd be rotated so that the plane of the Milky Way goes along the equator, the SGP is angled at 6-odd degrees to the horizontal (at the equator), and offset to the sides where it'd intersect the equator at roughly 137 and 137+180 degrees galactic longitude.

    And indeed, here's such one picture:
    And here's the same thing with coordinates drawn:

    The SGP is visible as the roughly ring-like structure that crosses the equator at approx 150 and 330 degrees.
    Remember that this is the Mollweide projection, similar to map projections of Earth globe of the same shape. It distorts shapes more and more towards the perimeter of the projection. So while SGP is a lopsided ring on the above two maps, it is a line (or more precisely, a great circle) running across the entire sky as viewed by an observer.
  10. Jan 8, 2019 #9
    Hey this is great, can you do me a small favour and just sketch that SGP on that pic you supplied? Just edit it with a simple editor and use circles or lines to show me that SGP. As you said this is a Mollweide projection , so its a little difficult to visualize.
  11. Jan 8, 2019 #10


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    Should be thereabouts:
  12. Jan 8, 2019 #11
    Fantastic! This will help me greatly! Where can I download a 3D map of the universe so I can use the data to illustrate this? What I want to do is grab 3D cartesian coordinates of all these super galactic markers that make the plane so I can put them in a 3D universe in the computer and then join all their points to make the plane visible and rotatable as a plane in 3D space.
  13. Jan 8, 2019 #12


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    Sorry, can you be more specific? Are you looking for a raw database of galaxies (such as from the 2MASS survey), or a 3D visualisation software like Digital Universe Atlas or similar?
  14. Jan 9, 2019 #13
    I want access to a database file with all the 3D coordinates of all these super clusters in X,Y,Z coordinates so I can make my own 3D visualisation software but make it different from other products, I want to do a few things that are a bit different:

    1. Apparent and real distances/locations. So my software will show where they appear to be but in a light delay compensation mode it will also be able to adjust their coordinates to take into account of the time for their light to reach us based on their observed velocity at the delayed time. So in the second mode it shows where they really are and not whey they appear to be. This can be done as I said with the velocity times the time it takes for the light to reach us but also take into account of their future trajectory and interactions with neighboring clusters as they also move in a O(N) computation body prediction problem. I have already got experience doing this with Runge Kutta.

    2. In this 3D environment the software will actually plot out the supergalactic plane as well as galactic plane so the user can see how they look like from an external perspective.

    But I would like both 3D and 2D coordinates so I can check for data consistency between the two and be able to use the 2D one to check the 3D one by plotting the 2D data on a sphere and then have each dot on the sphere vector out from the center of the sphere and see if they match up with the 3D coordinates.

  15. Jan 10, 2019 #14


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    The databases I'm aware of provide observational data in spherical coordinates only. This doesn't mean they don't exist, just that they're not sufficiently conspicuously out there for me to notice. You'll have to convert what they provide into Cartesian coordinates yourself. That shouldn't be too hard.
    Furthermore, they don't list masses nor transverse velocities, since those are not directly observable, and have to be inferred - if at all possible. This throws a wrench into your plans of building an N-body simulation, as you have neither the initial velocity (just the radial component) nor the mass for your particles.

    Having said that, maybe try this site:
    You can choose from various surveys. You'll want to display only galactic coordinates and distance (there are popups expanding the abbreviations in the tables).

    Also, you may want to watch this:

    (site: http://irfu.cea.fr/cosmography)
    and read the very accessible accompanying paper:
    They've done something like what you're thinking of doing, and talk in the paper about what catalogues they're using (after skimming the paper I think it's mostly V8k, available from the site linked above), discuss the data used, and what the video shows.
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